Conventionally, passive optical network (PON) systems have been widely employed as systems capable of realizing public networks using optical fibers. The PON system is one type of point-to-multipoint access optical communication systems.
The PON system is made up of one optical line terminal (OLT) and a plurality of optical network units (ONUs). The OLT is a station device and the ONUs are subscriber terminal devices. The OLT is connected to the respective ONUs via an optical star coupler. In the PON system, a large number of ONUs can share the OLT and most of the transmission lines (optical fibers). Lower operating cost can therefore be expected. Further, the optical star coupler is a passive component, which needs no power supply. The optical star coupler can therefore be installed outdoors easily. Further, the optical star coupler is high in reliability. Because of those reasons, the PON system has been actively introduced in recent years as a trump card for realizing a broadband network.
A gigabit Ethernet-passive optical network (GE-PON) system is now described as an example. In the GE-PON, the transmission rate is 1.25 Gbit/s as defined by the IEEE 802.3ah standard. The downlink from the OLT to the ONUs employs broadcast communications using a signal with an optical wavelength from 1,480 nm to 1,500 nm, and each of the ONUs extracts only data in an allocated time slot. The uplink from the ONUs to the OLT, on the other hand, employs broadcast communications using a signal with an optical wavelength from 1,260 nm to 1,360 nm, and time division multiplex communications in which transmission timings of data from the ONUs are controlled so as to prevent the collision of data.
Subsequently, 10G-EPON is described. In the 10G-EPON, the transmission rate is 10.3 Gbit/s as defined by the IEEE802.3av standard. In the 10G-EPON system, the downlink from the OLT to the ONUs employs broadcast communications using a signal with an optical wavelength from 1,574 nm to 1,580 nm, and each of the ONUs extracts only data in an allocated time slot. The uplink from the ONUs to the OLT, on the other hand, employs time division multiplex communications in which a signal with an optical wavelength from 1,260 nm to 1,280 nm is used and transmission timings of data from the ONUs are controlled so as to prevent the collision of data.
In the uplink communication of the above-mentioned PON system, the ONUs are positioned at different distances from the optical star coupler. The OLT therefore has different reception levels (received light levels) of the ONUs depending on received packets. Consequently, an OLT burst optical receiver needs to have wide dynamic range characteristics. The wide dynamic range characteristics as used herein are characteristics for reproducing burst signals having different reception levels stably at high speed. In general, the OLT burst optical receiver therefore includes an automatic gain control (AGC) circuit. The AGC circuit is a circuit for changing the conversion gain to a desired gain in accordance with the reception level.
The above-mentioned AGC circuit has a time constant required for the conversion gain to stably converge after the reception of a burst signal. Specifically, a predetermined time is necessary for the OLT burst optical receiver to reproduce data stably after receiving a burst signal. The predetermined time is standardized as a receiver settling time by IEEE802.3ah and IEEE802.3av. The standardized value of the receiver settling time is 400 ns under IEEE802.3ah (GE-PON system) and 800 ns under IEEE802.3av (10G-EPON system).
Each burst signal includes an overhead area and a data area. The overhead area has a length of the receiver settling time or larger. In order to improve the throughput of the overall system, however, the length of the overhead area is desired to be smaller. In this case, the AGC circuit operates based on a detection result of an average value of the received signals, and hence there is a trade-off relationship between the consecutive identical digit duration and the time constant. It is therefore a challenge to achieve both consecutive identical digit tolerance and high-speed response. Note that, the consecutive identical digit duration is the duration in which identical digits contained in a code string of the received signal are consecutive.
Various methods have been proposed for realizing an AGC circuit that has an excellent consecutive identical digit tolerance and responds at high speed. For example, in the methods described in Patent Literature 1 and Patent Literature 2, the AGC circuit is operated with a fast time constant only for the vicinity of the head of a received burst signal. After that, the AGC circuit is operated with a slow time constant.